The present invention relates to a fuel tank for an automobile, and more particularly to a fuel vapor discharge limiting device for limiting the communication between a fuel tank and a fuel vapor discharge pipe for discharging a fuel vapor in the fuel tank.
An automobile fuel tank is usually provided with a fuel vapor discharge pipe opening into an upper portion of the fuel tank. A fuel vapor in the fuel tank is introduced through the discharge pipe to a canister, and is adsorbed by the canister. In operating an engine, the fuel vapor thus adsorbed is separated from the canister by an intake vacuum of the engine, and is purged into an intake mixture gas.
As described in Japanese Utility Model Laid-open Publication No. 57-9615, for example, a float type discharge limiting valve is provided at an opening portion of the fuel vapor discharge pipe opening into the fuel tank. The float type discharge limiting valve is vertically moved in response to a change in liquid level in the fuel tank to close the opening portion of the discharge pipe when the liquid level is near a predetermined level. In supplying liquid fuel into the fuel tank, the communication between the fuel tank and the discharge pipe is cut off by the limiting valve to raise an internal pressure in the fuel tank to a given value which makes oversupply of the liquid fuel impossible, thus limiting the liquid level in the fuel tank to the predetermined full level.
FIG. 1 shows an example of such a discharge limiting valve in the prior art. Referring to FIG. 1, reference numeral 01 denotes an upper wail of a fuel tank. The upper wail is formed with an opening in which a cylindrical case 02 is mounted through a packing 03. The case 02 has a closed upper end with which a fuel vapor discharge pipe 04 is integrally formed. The closed upper end of the case 02 is formed with a communication hole 05 through which the discharge pipe 04 communicates with the inside of the case 02. The case 02 has an open lower end to which a bottom plate 07 having a central communication hole 06 is mounted. The inside of the case 02 communicates with the inside of the fuel tank through the communication hole 06.
A float 08 is accommodated in the case 02. The float 08 is supported through a spring 09 to the bottom plate 07. The float 08 is normally upwardly biased by the spring 09 so that the weight of the float 08 balances a biasing force of the spring 09. A valve seat 010 is formed at an opening of the communication hole 05 on the case 02 side, and a valve member 011 projects from an upper surface of the float 08 so as to be opposed to the valve seat 010.
When a liquid level in the fuel tank rises to come near a predetermined full level in supplying liquid fuel into the fuel tank, the liquid fuel enters the case 02 from the communication hole 06 to exert a buoyancy on the float 08. Accordingly, the float 08 is pushed up by the spring 09 until the valve member 011 of the float 08 comes into engagement with the valve seat 010. Thus, the communication hole 05 is closed by the valve member 011 of the float 08, so that an internal pressure in the fuel tank starts rising up to a given value as mentioned above, thus limiting the liquid level to the predetermined full level.
The internal pressure possibly rises according to various conditions such as an atmospheric temperature, fuel temperature, fuel property, and partial pressure of fuel vapor in the fuel tank. Such a rise in the internal pressure easily occurs especially when the liquid level in the fuel tank is the full level. In the prior art mentioned above, however, the communication hole 05 for making the communication between the fuel tank and the fuel vapor discharge pipe 04 is closed by the valve member 011 of the float 08 in the condition of the full level, and there is no means for releasing the increased internal pressure in this condition. Accordingly, there is a possibility that the internal pressure may excessively rise over a permissible value in the fuel tank. Further, there is another possibility that such a rise in the internal pressure may occur just after the fuel tank is filled to cause splash of the liquid fuel from a fuel filler inlet.